Things didn’t go quite as planned for Heidi Montag in the baby bump department. “I thought I would gain maybe 20 or 30 pounds during my pregnancy,” says Montag in the new issue of Us Weekly. “But I gained over 45.” That made the February day when she slipped into her “first pair of jeans that weren’t maternity,” even more thrilling. “It was very exciting,” the new mom tells Us. “It made me realize I’d lost a lot of weight.”

Not that she was trying. Since welcoming son Gunner with husband Spencer Pratt on October 1, The Hills alum tells Us she’s been so busy caring for her newborn that she “wasn’t really concerned with getting in shape right away. I wanted to give it time.”

As the 31-year-old notes, “I just had a baby, I’m very emotional.” The 5-foot-2 beauty, wed to former costar Pratt, 34, since 2009, had also begun a de facto weight-loss plan the minute she met her son. “I’m eating healthier than when I was pregnant, ironically, because I want to make sure Gunner has all the nutrients he needs,” says the breast-feeding mom. Parenthood has also changed Heidi’s thinking on beauty: “Having a baby has made me really appreciate my body, not just how it looks. It’s just amazing, the human body.”

Heidi PrattMaddie Córdoba

The reality star — who underwent 10 plastic surgery procedures in one day in 2010 (she later removed her size F breast implants) — no longer understands that nip/tuck mind-set. “I have no temptation to have any kind of surgery after having a baby,” she says. “I want to breast-feed and have my breasts for that.” She expects that feeling to stay. “Maybe I would get my boobs done after all the kids we have,” she muses. “But I might just be over that phase of my life.”

For now she’s marking a huge victory. Having dropped the weight she gained with Gunner (“Maybe even more!”), she estimates she’s at 107 pounds. “I just eat whatever I want here and there. I try to make sure I get enough food instead of limiting food.” On the menu: a protein at every meal, along with some carbs, and vegetables twice a day. Says Montag, “I pretty much eat the same thing every day.”

A “very encouraging” Pratt (“He’s like, ‘You always look great,’” she notes) has been serving up most of her meals. Montag admits she hasn’t cooked much since Gunner’s arrival, and her husband has picked up the slack: “He is the best griller. He’s a miraculous chef.”

With Pratt in the kitchen, all of Montag’s time can go to her “very smart” 5-month-old — who has yet to sleep through the night. “I feel a little crazy. I’m like the zombie emoji,” says the hands-on mom. “I change all the diapers; I don’t want to miss one!” Still, Pratt manages to do his part. When Gunner cries at 3 a.m., she says “he’ll just rub my shoulders. He always asks if I want him to get up with Gunner instead and give him a bottle.” After all, their father-son bond runs deep. “Spencer is an amazing dad,” says Montag. “He’s very fun, interactive and loving. They have their own little language.”

Meanwhile, Montag is learning a new appreciation for her capabilities. “My favorite body part is probably my stomach. Since I’ve just had a baby in there, it’s more meaningful to me,” she says. “It’s fun to show off my mom bod, the tiger stripes.”

When the new mom needs tips, she runs for The Hills! Mother of three Kristin Cavallari “gave me the best mom advice,” says the star. “That was just to listen to yourself and every kid is different, so you can’t go off all these books.” And Audrina Patridge, mom of 20-month-old Kirra, offered encouragement, noting “how quick it will all go.” Now, Gunner’s just waiting for his first playdate with Lauren Conrad’s 8-month-old son. “If she wants to get Liam and Gunner together,” says Montag. “I’m all for it.”

For more inside Montag’s life as a mom, grab the new issue of Us Weekly, on newsstands now.